The present invention relates to a system and process for dendritic web crystal growth. More particularly, the present invention relates to a system and process for stabilizing dendritic web crystal growth.
It has long been recognized that dendritic web ribbon crystals lend themselves as nearly ideal substrates for solar cells because of their high chemical purity, low density of structural defects, rectangular shape and relatively thin crystal size. Furthermore, solar cells fabricated from dendritic web silicon enjoy light energy to electrical energy conversion efficiencies as high as 17.3%, which is comparable to high efficiencies obtained using expensive processes such as Float Zone silicon and other well known complex processes.
FIG. 1 shows a diagram of a dendritic web silicon crystal 10 in the form of a silicon ribbon or sheet emerging as a single crystal from a silicon melt 14 contained in a crucible 12. In order to solidify the web silicon crystal 10 during the crystal growth process, silicon melt 14 is maintained a few degrees below the freezing point of silicon (1412xc2x0 C.) inside crucible 12. Silicon crystal 10 is typically grown by pulling upward on a top dendrite seed 22 at a speed of approximately 1.5 cm/min. The resulting dendritic web silicon crystal 10 includes a silicon web portion 16 bounded by silicon dendrites 18. The web portion 16 is typically about 3 to 6 cm in width and about 100 xcexcm in thickness compared to the nominally square dendrites 18, which are typically about 700 xcexcm thick. In order to sustain the above described crystal growth, the dendrite support structure should be continually regenerated at the pointed dendrite tips 20 beneath the surface of silicon melt 14.
Unfortunately, the conventional dendritic web crystal growth processes suffer from several drawbacks. By way of example, conventional dendritic web crystal growth processes are difficult to commercialize because they are xe2x80x9cmetastablexe2x80x9d and subject to premature termination of crystal growth. Although, in rare instances a dendritic web crystal longer than about 5 m and having a width that is between about 3 and about 6 cm may be grown, minor random perturbations in the growth environment frequently prematurely terminate crystal growth. As a result, most crystals, according to conventional methods, cease growing typically after 1-2 hours when the crystals are of lengths that are between about 1 and about 2 m or far less than a commercially desirable length of 5 m or higher. Thus, conventional crystal growth techniques fail to reproducibly provide sufficiently long crystals.
As another example, added costs and wasted time associated with premature termination of crystal growth make the conventional web crystal growth process undesirable. After the premature termination of crystal growth, it takes an operator 1 to 2 hours to configure the dendritic web crystal growing system to start growing the next crystal. Consequently, valuable labor costs and time are expended to begin crystal growth again.
As yet another example, when conventional web crystal growth techniques are employed, most crystals grow in transient, rather than steady-state conditions. A crystal starting out at a width of about 3 cm gradually widens due to transient conditions to a value that is between about 5 and about 6 cm over several meters of length. The completed crystal must be trimmed so as to have a consistent width along the entire length. Thus, solar cells that are currently fabricated from web crystal ribbons produced by conventional techniques are done so at the expense of valuable excess web crystal surface that is wasted.
What is, therefore, needed is a system and method for stabilizing dendritic web crystal growth that can be commercialized without suffering from the drawbacks of the conventional methods described above.
The present invention provides an improved system and process of dendritic web crystal growth, which substantially overcomes the above-noted problems of premature termination of crystal growth. In one aspect, the present invention provides a process for dendritic web growth that includes: providing a melt; growing a dendritic web crystal from the melt; replenishing the melt during the step of growing the dendritic web crystal; and applying a magnetic field to the melt during the step of growing the dendritic web crystal.
The dendritic web crystal of the present invention may be a silicon or a germanium crystal. The melt employed in one embodiment of the present invention, therefore, includes at least one material selected from the group consisting of silicon and germanium. In another embodiment of the present invention, the melt further includes tin.
The step of applying the magnetic field to the melt may include providing a magnetic field strength that allows dendrites that support the web crystal to be continually regenerated underneath a surface of the melt. The magnetic field strength may generally be greater than or equal to about 400 Gauss and may preferably be between about 400 and about 2500 Gauss.
In accordance with one embodiment of the present invention, the step of growing includes pulling a silicon seed crystal from the melt. The dendritic web crystal is pulled at a rate that is generally greater than or equal to about 1.5 cm/min, and preferably greater than or equal to about 1.8 cm/min, to ensure that growth of the dendritic web silicon crystal does not prematurely cease.
The step of replenishing the melt may include delivering silicon pellets to the melt. The pellet delivery rate is generally greater than or equal to 0.20 g/min and is preferably greater than or equal to 0.40 g/min.
In one embodiment of the present invention, the step of applying the magnetic field includes producing a magnetic field that is oriented perpendicular to the plane of the web crystal. Alternatively, in another embodiment of the present invention, the magnetic field is oriented parallel to the plane of the web crystal in the horizontal direction. In yet another embodiment of the present invention, the magnetic field is in the vertical direction and perpendicular to the plane of the melt.
In another aspect, the present invention provides an apparatus for dendritic web growth. The apparatus includes: (1) a crucible including a feed compartment for receiving pellets to facilitate melt replenishment and a growth compartment designed to hold a melt for dendritic web growth; and (2) a magnetic field generator configured to provide a magnetic field during dendritic web growth.
The apparatus of the present invention further includes a growth furnace and the crucible mentioned above is disposed within the growth furnace. The magnetic field generator, according to one embodiment of the present invention, includes an electromagnet or a permanent magnet, which is mounted outside the growth furnace. The magnetic field generator of the present invention may be a superconducting magnet that is mounted outside the growth furnace. In embodiments where magnet pole pieces serve as magnetic field generators of the present invention, the magnet pole pieces include at least one portion that is located outside the growth furnace.
In accordance with one embodiment of the present invention, the magnetic field generator is configured to produce a magnetic field that is oriented perpendicular to the plane of the web crystal and a power consumed by the magnetic field generator to produce the magnetic field of sufficient strength is reduced. Alternatively, in another embodiment of the present invention, the magnetic field generator is configured to produce a magnetic field that is oriented in the horizontal direction and parallel to the plane of the web crystal. The magnetic field generator may also be configured to produce a magnetic field that is in the vertical direction and perpendicular to the plane of the web melt.
In yet another aspect, the present invention provides a dendritic web crystal fabricated using a process which includes: providing a melt; growing a dendritic web crystal from the melt; replenishing the melt during the step of growing the dendritic web crystal; and applying a magnetic field to the melt during the step of growing the dendritic web crystal. The step of applying the magnetic field to the melt may include providing a magnetic field strength that allows dendrites that support the web crystal to be continually generated underneath a surface of the melt.
These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures.